Help with DataGridViewComboBox column

Hello experts.  I am having trouble with a DataGridViewComboBox column.  I am using unbound mode.  I need to give the user a specifc list of dates to choose from based on the selected quarter.  


Here is the source datatable for the combo column:

(I have StartDate and EndDate columns.  I am only showing StartDate here for simplicity.  EndDate is the same.)

m_dtAdjDates:
ComboID      StartDate     EndDate
   0              NULL        NULL
   1        06/30/2008  07/28/2008
   2        07/28/2008  09/01/2008
   3        09/01/2008  09/29/2008



The way I understand it, the combobox must contain every possible value that might be found in the underlying data.  For now, all of the records in my test data contain NULL in the AdjStartDate column.  So the combo just needs to be defaulted to item zero, which definitely exists in the combo source table.


Here is how I have created the combo column in the grid:

Dim col As DataGridViewComboBoxColumn = New DataGridViewComboBoxColumn()
With col
    .DataPropertyName = "AdjStartDate"
    .HeaderText = "Adjustment Start Date"
    .Name = "AdjStartDate"
    .DefaultCellStyle.Format = "MM/dd/yyyy"
    .MaxDropDownItems = 4
    .FlatStyle = FlatStyle.Flat
    .DisplayStyle = DataGridViewComboBoxDisplayStyle.Nothing

    .DataSource = m_dtAdjDates
    .ValueMember = "ComboID"
    .DisplayMember = "StartDate"

End With
grd.Columns.Add(col)



Here is how I populate the grid.  As I mentioned above, I just need to default the combobox to item zero.

Private Sub getData()
    Dim dt As DataTable
    Dim i As Integer
    Dim k As Integer
    Dim q As Integer
    Dim dTmp As Date

    dt = frmMain.m_bll.opReportedSubcounts_getAdjustments(m_iBCYear, m_iBCMonth, cmbZones.Text)

    grd.Rows.Clear()

    For i = 0 To dt.Rows.Count - 1

        grd.Rows.Add()
        k = grd.Rows.Count - 1

        'Populating other columns...
        '...


        If dt.Rows(i).Item(ColumnEnum.adj_start_date) Is DBNull.Value Then

            '*******************************************************************
            '*******************************************************************
            '*******************************************************************
           
            'This code is causing an error, "DataGridViewComboBoxCell value is not valid"

            grd.Rows(k).Cells(ColumnEnum.adj_start_date).Value = 0

            '*******************************************************************
            '*******************************************************************
            '*******************************************************************
        Else

            '*******************************************************************
            '*******************************************************************
            '*******************************************************************

            'This code will come into play later when I get some dates in the
            '  the column. But for now, you can ignore it as all records are NULL.

            dTmp = dt.Rows(i).Item(ColumnEnum.adj_start_date)
            For q = 1 To m_dtAdjDates.Rows.Count - 1
                If m_dtAdjDates.Rows(q)("StartDate") = dTmp Then
                    grd.Rows(k).Cells(ColumnEnum.adj_start_date).Value = q
                    Exit For
                End If
            Next q

            '*******************************************************************
            '*******************************************************************
            '*******************************************************************

        End If


        'Populating other columns...
        '...

    Next i

    dt = Nothing

    grd.Refresh()

End Sub




Any help is appreciated.
Thanks.